In the field of imaging, the role of the scintillator is to convert ionizing radiation into visible light radiation. To produce an imaging device of good quality, two opposing parameters must be optimized:                The detection threshold which imposes maximization of the scintillator thickness to increase the probability of interaction of the radiation with the scintillator and thus the light yield from conversion.        The spatial resolution which requires minimization of the scintillator thickness to limit the scattering of visible light since luminescence is an isotropic phenomenon.        
Another parameter is the speed of scintillation decay; the lower it is, the better it enables rapid phenomena to be followed.
In practice, the material constituting a scintillator must be transparent, and emit as far as possible in the visible region, preferably above 500 nanometers (in fact, what is most important is for the scintillator to emit radiation that is compatible with a detector with which it may be required to be associated).
Two broad families of scintillators are known, according to whether the material of which they are constituted is inorganic or organic.
Within the field of ionizing radiation, the majority of commercial scintillators are mass single-piece inorganic crystals such as for example bismuth germanate (BGO) or cerium-doped yttrium aluminum garnet (YAG:Ce). As a result of their high density, these scintillators retain a relatively moderate thickness, while ensuring a high probability of interaction.
When it is sought to increase the sensitivity, it is necessary to turn to significant scintillator thicknesses. It is then necessary to use segmentation techniques in order to retain a satisfactory spatial resolution (the scintillator is known as “segmented” or “structured”). The scintillator is then constituted by a multitude of fibers, having for example a parallelepipedal cross section, the length of which is much greater than the other dimensions. These fibers are bonded together and optically isolated, oriented in such a way that the incident radiation arrives parallel to the largest dimension of the needles. In this case, the degradation of spatial resolution is limited because the light produced in each fiber cannot propagate into the adjacent fibers. Typically, the fibers have a length of the order of a centimeter and the other dimensions are at least several hundreds of micrometers.
However, this segmentation technique is extremely expensive and can only be applied to certain materials the crystal growth of which can be controlled.
Moreover, inorganic scintillators, due to their crystalline structure, have a response time that is too long for some applications. In fact, the response time of these materials in the great majority of cases greatly exceeds 10 ns, which is incompatible with some uses, in particular in diagnostics with fast laser X-ray imaging.
As regards organic scintillators, they have been used for a long time for ionizing radiation imaging. Reference may be made in particular to U.S. Pat. No. 4,495,084, (or the priority French Pat. No. FR 2 511 387), or to European Patent Application No. EP 0913448 or to PCT Application Publication No. WO 2014/135640. To the extent that the interception of the radiation often induces a scintillation at small wavelengths (outside the visible spectrum), it is known to incorporate molecules, called fluorophores, having the effect of converting the radiation resulting from the scintillation into radiation of longer wavelength, situated within the visible spectrum.
Nevertheless, with ionizing radiation, these organic scintillators have a lower probability of interaction than the inorganic scintillators, due to the low atomic number Z of the organic components. In order to overcome this difficulty, two broad techniques are used:                Filling a capillary matrix with the scintillation material, having the effect of reducing the light dispersion and allowing the use of large scintillator thicknesses, or        Adding a high-Z material to the scintillation material in order to increase the density thereof.        
Among the capillary scintillators there may be mentioned for example, filling a matrix of glass capillaries with a liquid scintillation material as described in PCT Application Publication No. WO 03/081279; these are for example capillaries the inner diameter of which is less than or equal to 50 micrometers, for example equal to 20 micrometers; with regard to the scintillation liquid, it contains a large fraction of deuterium. Nevertheless, its exploitation is difficult and complicated, because it is necessary to ensure that the liquid is confined, in particular under vacuum, which involves the use of a transparent confining material in order to allow the light to exit, which has the double drawback of increasing the number of optical interfaces (and consequently increasing the transmission losses, which is detrimental to the signal to be detected) and preventing any thermal expansion of the liquid scintillator (which increases the internal pressure in the capillaries and can lead to the destruction of the matrix).
Mention may also be made of filling a matrix of glass capillaries with a monomer and a neutron-absorbing material followed by a polymerization process; a first attempt was proposed in 1985 [E. Bigler, F. Ploack, Applied Optics, Vol. 24, No. 7, 994-997]; more recently it was proposed in U.S. Pat. No. 7,372,041 to fill capillaries of a plastic material or of glass having a diameter comprised between 10 and 200 micrometers with an organic polymer such as polystyrene or polyvinyltoluene, doped with a small percentage of Li, B, Sm, Cd, Eu, Gd or Dy, possibly also containing fluorophores. But this solution is limited to the detection of the neutrons and does not apply to ionizing radiation imaging.
It should be noted that the formation of a structured organic scintillator, comprising a network of small-diameter capillaries, involves in practice being able to form a homogeneous mixture in a mold in which pressure is reduced so as to force the mixture to penetrate into the capillaries by suction; these molds are therefore more complex than those in which monolithic scintillators are formed, which can be used at atmospheric pressure. It is understood that this penetration is made easier, the more homogeneous the viscosity, and the mixture, it being noted that it is advantageous to be able to make this mixture and to force its penetration into the capillaries at ambient temperature, and not in an enclosure at a controlled temperature above said ambient temperature. Next, polymerization of the mixture is induced, which is carried out in practice by increasing the temperature; it is self-evident that the operations of mixing and forced penetration of the mixture must not be carried out at a temperature at which polymerization of the mixture can take place.
The other option consisting of adding a high-Z material, such as lead, into an organic scintillator has been known since the 1950s [Pichat, L., Pesteil, P., Clément, J. J., Chim. Phys. 1953, 50, 26-41 and Lin, Q.; Yang, B.; Li, J.; Meng, X.; Shen, J. Polymer 2000, 41, 8305-8309]. This improves the interaction with the radiation and therefore the sensitivity of the scintillator. Nevertheless, high-Z materials can only be incorporated at low percentages and homogeneity of the mixture is very difficult to obtain, due to the precipitation phenomena observed with molecules containing heavy elements. More recently, some authors (M. Hamel, G. Turk, A. Rousseau, S. Darbon, C. Reverdin, S. Normand, Nucl. Instr. and Meth. A 660 (2011) 57-63 and PCT Application Publication No. WO 2012/085004 to M. Hamel, S. Darbon, S. Normand, and G. Turk) have shown that, by using an organometallic compound called lead dimethacrylate as a cross-linking agent of vinyltoluene, and methacrylic acid, it was possible to reach a high level of incorporation of the lead, ranging up to 12.3% (with 2-hydroxyethyl methacrylate instead of methacrylic acid, it was even possible to obtain 29%). An important aspect is that the lead atom is grafted directly onto the polymer chain, which ensures that a homogeneous distribution thereof is maintained as soon as the cross-linking begins, and avoids an accumulation by precipitation (PCT Application Publication No. WO 2012/085004 to M. Hamel, S. Darbon, S. Normand, and G. Turk). These authors add fluorophores to the scintillating organic material in order to shift the light emission to wavelengths suitable for the image sensor.